I can't believe you just made me do that. I ALMOST PEED. But I'll have to go with Voltron seeing as there's one for each area of the universe, which in turn means there are five separate mechs AND pilots for each part of the universe. There just aren't that many Thundercats.
I say Thundercats for the reason that they never had a reason to have a huge Mecha for battle. Now if the time arose for such battle. I am more than sure that Panthro and Bengali would team up to whip some badass Thundranium powered mecha that would whup Voltron seven ways to sunday. Plus if it were a one on one match up... the Cats would claw those kids to pieces. Thundercats all the way.